User`s guide

The pull-up resistor provides a reference to +5V while its value of 2200 ohms
requires only 2.3 mA of drive current
If the 82C55 is reset and enters high impedance input, the line is pulled high. At that
point, both the 82C55 AND the device being controlled will sense a high signal.
If the 82C55 is in output mode, the 82C55 has more than enough power (2.5mA) to
over ride the pull-up/down resistor's high signal and drive the line to 0 volts. If the
82C55 asserts a high signal, the pull up resistor guaranties that the line goes to +5V.
Of course, a pull-down resistor accomplishes the same task except that the line is
pulled low when the 82C55 is reset. The 82C55 has more than enough power to drive
the line high.
The PCI-DIO24 series boards are equipped with positions for pull-up/down resistors
Single Inline Packages (SIPs). The positions are marked A, B and C and are located
beside the 82C55 or output chips.
A 2.2K, 8 resistor SIP is made of 8, 2.2K resistors all connected one side to a single
common point and the other, each to a pin protruding from the SIP. The common line
to which all resistor are connected also protrudes from the SIP. The common line is
marked with a dot and is at one end of the SIP.
The SIP may be installed as pull-up or pull-down. At each location, A, B & C on the
PCI-DIO24 series boards, there are 10 holes in a line. One end of the line is +5V, the
other end is GND. They are so marked. The 8 holes in the middle are connected to
the 8 lines of the port, A, B, or C.
A resistor value of 2.2K is recommended. Use other values only if you have calcu-
lated the necessity of doing so.
UNCONNECTED INPUTS FLOAT!
Keep in mind that unconnected inputs float. If you are using the DIO24 board for
input, and have unconnected inputs, ignore the data from those lines.
In other words, if you connect bit A0 and not bit A1, do not be surprised if A1 stays
low, stays high or tracks A0... It is unconnected and so unspecified. The 82C55 is
not malfunctioning. In the absence of a pull-up/down, any input to a PCI-DIO24
which is unconnected, is unspecified!
You do not have to tie input lines, and unconnected lines will not affect the perform-
ance of connected lines. Just make sure that you mask out any unconnected bits in
software!
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